Learning Kotlin - An Android Developers Journey

This presentation was presented at the Kotlin Johannesburg meetup.

Learning a new programming language is a daunting thought. It is not an overnight accomplishment, where the next day you are suddenly well versed in the new language. Rebecca is in the process of learning Kotlin. She has often felt like the biggest imposter trying to learn this new language that is growing in popularity. She has had thoughts like: “But there are so many experts out there, I’ll never know as much as them.”, “How can I start writing Kotlin when I don’t know all the features there are in the language?”. The biggest lesson she has learned about learning Kotlin is to just write it. In this talk, she will cover some of the nifty features she has explored in Kotlin, as well as the many different ways she has gone about trying to learn this new language, slowly overcoming her imposter syndrome whilst writing Kotlin code.

Find Rebecca Franks on Twitter: https://twitter.com/riggaroo

Transcript

1. Learning Kotlin
   An Android Developer’s Journey
   Rebecca Franks
   @riggaroo
   

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2. What is Kotlin?
   

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3. What is Kotlin?
   

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4. Why Learn Kotlin?
   

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5. My Journey
   

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6. I saw general hype about
   Kotlin...
   

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7. I watched a video...
   

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10. I wrote an article...
    

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13. Then I forgot about it..
    

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14. Google I/O happened
    

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15. Google I/O 2017 - Kotlin Support Announcement
    

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16. In my mind
    

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17. I tried Kotlin Koans...
    

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18. try.kotlinlang.org
    

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19. I tried an Online Course
    

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20. Udemy
    

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21. I bought a Kotlin book
    

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23. I bought another Kotlin Book…
    

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25. I decided to CMD +ALT+ SHIFT + K
    a few files
    

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27. Eventually I decided to just try
    write Kotlin code
    

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30. You don’t need to know all the features
    to write Kotlin code.
    Learn as you go.
    

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31. Kotlin Things
    

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32. Semicolons - there are none
    *well unless you want them
    

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33. var / val
    

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34. var = variable value
    

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35. val = constant value
    

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36. var temperature : Int = 1 // has type Int
    val numberMembers = 5 //automatically inferred type Int
    val percentage = 80 / 100
    percentage = 30 //does not compile
    var percentage = 80 / 100
    percentage = 30 //compiles
    

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37. var temperature : Int = 1 // has type Int
    val numberMembers = 5 //automatically inferred type Int
    val percentage = 80 / 100
    percentage = 30 //does not compile
    var percentage = 80 / 100
    percentage = 30 //compiles
    

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38. var temperature : Int = 1 // has type Int
    val numberMembers = 5 //automatically inferred type Int
    val percentage = 80 / 100
    percentage = 30 //does not compile
    var percentage = 80 / 100
    percentage = 30 //compiles
    

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39. var temperature : Int = 1 // has type Int
    val numberMembers = 5 //automatically inferred type Int
    val percentage = 80 / 100
    percentage = 30 //does not compile
    var percentage = 80 / 100
    percentage = 30 //compiles
    

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40. Nullability
    

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41. The Kotlin Type system distinguishes
    between references that can hold null
    and those that can not
    

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42. Avoid NullPointerException
    *well unless you want them
    

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43. //No Nulls Allowed
    var average : Double = 1.0
    average = 2.0
    average = null
    

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44. //No Nulls Allowed
    var average : Double = 1.0
    average = 2.0
    average = null
    //Allows nulls
    var average : Double? = 1.0
    average = 2.0
    average = null
    

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45. // Double !! -> NullPointerException
    val l = name!!.length
    // Safe Call ?. with chains
    val name = bob?.department?.head?.name
    // Elvis Operator
    val length = b?.length ?: -1
    

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46. // Double Bang !! -> NullPointerException
    val l = b!!.length
    // Safe Call ?. with chains
    val name = bob?.department?.head?.name
    // Elvis Operator
    val length = b?.length ?: -1
    

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47. // Double Bang !! -> NullPointerException
    val l = b!!.length
    // Safe Call ?. with chains
    val name = bob?.department?.head?.name
    // Elvis Operator ?:
    val length = b?.length ?: -1
    

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48. val name : String? = "hello"
    name?.let {
    //Now name is not null at this point
    textView.text = name
    }
    // There are whole bunch more - .with{} .run{} - with slight differences
    // Can also just do a normal null check!
    

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49. lateinit
    

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50. class EventsActivity : AppCompatActivity() {
    private lateinit var recyclerViewAdapter: SimpleAdapter
    override fun onCreate(savedInstanceState: Bundle?) {
    // Other initialization stuff
    recyclerViewAdapter = SimpleAdapter(/*..*/)
    }
    }
    

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51. lazy
    

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52. val lazyValue: String by lazy {
    println("computed!")
    "Hello"
    }
    fun main(args: Array) {
    println(lazyValue)
    println(lazyValue)
    }
    computed!
    Hello
    Hello
    

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53. when
    

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54. when (s) {
    1 -> print("x == 1")
    2 -> print("x == 2")
    else -> { // Note the block
    print("x is neither 1 nor 2")
    }
    }
    

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55. enum class Currency {
    GBP,
    ZAR;
    }
    when (currency) {
    Currency.GBP -> doSomething()
    Currency.ZAR -> doAnotherThing()
    }
    

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56. enum class Currency {
    GBP,
    USD,
    ZAR;
    }
    when (currency) {
    Currency.GBP -> doSomething()
    Currency.ZAR -> doAnotherThing()
    }
    

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57. enum class Currency {
    GBP,
    USD,
    ZAR;
    }
    when (currency) {
    Currency.GBP -> doSomething()
    Currency.ZAR -> doAnotherThing()
    else -> doNothing()
    }
    

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58. Functions
    

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59. Functions are first-class citizens
    

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60. fun calculateSum(a: Int, b: Int) : Int {
    return a + b
    }
    fun calculateSum(a: Int, b: Int) = a + b
    

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61. fun calculateSum(a: Int, b: Int) : Int {
    return a + b
    }
    fun calculateSum(a: Int, b: Int) = a + b
    

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62. Extension Functions
    

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63. fun String.hello(): String = "Hello " + this
    // use the new hello() method
    val hi = "Kotlin !".hello()
    Hello Kotlin !
    fun TextView.getFloatValue() : Float {
    return 0f
    }
    val float = textViewHello.getFloatValue()
    

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64. fun String.hello(): String = "Hello " + this
    // use the new hello() method
    val hi = "Kotlin !".hello()
    Hello Kotlin !
    fun TextView.getFloatValue() : Float {
    return 0f
    }
    val num = textViewHello.getFloatValue()
    

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65. Default Values
    

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66. fun calculateSum(a: Int = 2, b: Int) : Int {
    return a + b
    }
    val sum = calculateSum(b = 3)
    5
    //Can be done on Constructors as well as functions
    

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67. Higher Order Functions & Lambdas
    

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68. val exampleArray = arrayListOf(3,324,5,6,3,2,6,8,1,2,4,3,2)
    exampleArray.filter { it % 2 == 0 }
    [324, 6, 2, 6, 8, 2, 4, 2]
    val numberList = arrayListOf(1,2,3,4,5,6,7,8,9)
    numberList.map { it * 2 }
    .filter { it % 3 == 0}
    [6, 12, 18]
    

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69. val exampleArray = arrayListOf(3,324,5,6,3,2,6,8,1,2,4,3,2)
    exampleArray.filter { it % 2 == 0 }
    [324, 6, 2, 6, 8, 2, 4, 2]
    val numberList = arrayListOf(1,2,3,4,5,6,7,8,9)
    numberList.map { it * 2 }
    .filter { it % 3 == 0}
    [6, 12, 18]
    

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70. fun calcPercentage(value: Int, onComplete: (output : Double) -> Unit) {
    val calculation = value / 100.0
    onComplete(calculation)
    }
    //called in onCreate or wherever
    calculatePercentage(2) {
    Log.d("MainActivity", "Calculation Complete: $it")
    }
    Calculation Complete: 0.02
    

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71. fun calcPercentage(value: Int, onComplete: (input : Double) -> Unit) {
    val calculation = value / 100.0
    onComplete(calculation)
    }
    //called in onCreate or wherever
    calcPercentage(2) {
    Log.d("MainActivity", "Calculation Complete: $it")
    }
    Calculation Complete: 0.02
    

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72. fun executeWork(job: () -> T) : T {
    Log.d(TAG, "Work started")
    val result = job()
    Log.d(TAG, "Work Fin")
    return result
    }
    val number = executeWork { 100 * 5 }
    Log.d(TAG, "Result: $number")
    val stringCalc = executeWork { "Rebecca".plus(" Hello") }
    Log.d(TAG, "String: $stringCalc")
    Work started
    Work Fin
    Result: 500
    Work started
    Work Fin
    String: Rebecca Hello
    

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73. fun executeWork(job: () -> T) : T {
    Log.d(TAG, "Work started")
    val result = job()
    Log.d(TAG, "Work Fin")
    return result
    }
    val number = executeWork { 100 * 5 }
    Log.d(TAG, "Result: $number")
    val stringCalc = executeWork { "Rebecca".plus(" Hello") }
    Log.d(TAG, "String: $stringCalc")
    Work started
    Work Fin
    Result: 500
    Work started
    Work Fin
    String: Rebecca Hello
    

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74. fun executeWork(job: () -> T) : T {
    Log.d(TAG, "Work started")
    val result = job()
    Log.d(TAG, "Work Fin")
    return result
    }
    val number = executeWork { 100 * 5 }
    Log.d(TAG, "Result: $number")
    val stringCalc = executeWork { "Rebecca".plus(" Hello") }
    Log.d(TAG, "String: $stringCalc")
    Work started
    Work Fin
    Result: 500
    Work started
    Work Fin
    String: Rebecca Hello
    

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75. Classes
    

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76. class Admin : User {
    val name: String
    val age: Int
    constructor(name : String, age : Int){
    this.name =name
    this.age = age
    }
    fun isAdult(): Boolean {
    return age >= 18
    }
    }
    

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77. class Admin(val name: String, val age: Int) : User {
    fun isAdult(): Boolean {
    return age >= 18
    }
    }
    

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78. class Admin(val name: String, val age: Int) : User {
    fun isAdult() = age >= 18
    }
    

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79. Data Classes
    

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80. // data classes
    data class Admin(val name: String, val age: Int)
    // Automatically generates:
    - toString()
    - hashCode() / equals()
    - copy()
    

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81. Coroutines
    *Experimental
    

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82. Simplify Async Programming
    Lightweight thread
    *Spin up 1mil coroutines easily
    

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83. Coroutines are computations that can be
    suspended without blocking a thread.
    https://kotlinlang.org/docs/tutorials/coroutines-basic-jvm.html
    

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84. //onCreate
    job = launch(background) {
    val result = requestData()
    launch(UI) {
    textViewHello.text = result
    }
    }
    //suspend function
    suspend fun requestData(): String {
    delay(3000)
    return "DONE"
    }
    //onDestroy
    job.cancel()
    

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85. //onCreate
    job = launch(background) {
    val result = requestData()
    launch(UI) {
    textViewHello.text = result
    }
    }
    //suspend function
    suspend fun requestData(): String {
    delay(3000)
    return "DONE"
    }
    //onDestroy
    job.cancel()
    

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86. //onCreate
    job = launch(background) {
    val result = requestData()
    launch(UI) {
    textViewHello.text = result
    }
    }
    //suspend function
    suspend fun requestData(): String {
    delay(3000)
    return "DONE"
    }
    //onDestroy
    job.cancel()
    

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87. https://kotlinlang.org/docs/tutorials/c
    oroutines-basic-jvm.html
    

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88. Android KTX
    

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89. github.com/android/android-ktx
    

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90. A set of Kotlin extensions for Android
    

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91. //Before
    sharedPreferences.edit()
    .putBoolean("key", value)
    .apply()
    //After - with Android KTX
    sharedPreferences.edit {
    putBoolean("key", value)
    }
    

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92. //Before
    view.viewTreeObserver.addOnPreDrawListener(
    object : ViewTreeObserver.OnPreDrawListener {
    override fun onPreDraw(): Boolean {
    viewTreeObserver.removeOnPreDrawListener(this)
    actionToBeTriggered()
    return true
    }
    })
    //After - with Android KTX
    view.doOnPreDraw {
    actionToBeTriggered()
    }
    

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93. val (a,r,g,b) = Color.MAGENTA
    val drawable = customDrawable.toBitmap()
    canvas.withTranslation(x, y) { // block }
    val (year, month, day) = LocalDate.now()
    

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94. The difficult bits
    *for me...
    

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95. Mocking
    - All classes in Kotlin are final unless you tell it otherwise (ie mark it as
    open )
    - Workarounds but seem unnecessary
    https://antonioleiva.com/mockito-2-kotlin/
    http://hadihariri.com/2016/10/04/Mocking-Kotlin-With-Mockito/
    

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96. Tooling
    IntelliJ is great… but it doesn’t write all the code.
    Battled with Lambda syntax, it sometimes doesn’t fill in all the blanks
    nicely
    Solution - Write it in Java, then copy paste the java into your Kotlin file
    and it’ll auto convert
    

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97. Nothing, Unit & Any
    Nothing - never returns - ie infinite loop or exception throwing
    Unit - default return type for a function
    Any - Default Super Type (similar to Java’s Object type)
    http://natpryce.com/articles/000818.html
    

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98. Concerns
    ● Packaging another library in your app (size 9kbs when proguarded - not
    much)
    ● Another barrier to entry for new developers
    ● Java 8 works with Android.. (not all features ie Stream API only available
    in 24+)
    ● Certain Java plugins not supported (yet) - Sonarqube
    https://discuss.kotlinlang.org/t/sonarqube-support/3657
    

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99. Resources
    

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100. Online Courses
     O’Reilly Introduction to Kotlin
     YouTube Search “Kotlin on Android”
     https://www.youtube.com/results?search_query=Kotlin+on+Android
     

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101. Practise
     Kotlin Koans https://try.kotlinlang.org
     Kotlin REPL
     Kotlin Android Codelab
     

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102. try.kotlinlang.org
     

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105. Books
     ● Kotlin in Action
     - Svetlana Isakova
     - Dmitry Jemerov
     ● Kotlin for Android Developers
     - Some common patterns etc
     

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106. Online Community
     kotlinweekly.net
     talkingkotlin.com
     reddit.com/r/kotlin
     

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107. Just start writing Kotlin code
     

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108. Thank you!
     Questions?
     Rebecca Franks
     @riggaroo
     

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